R-T-B-based alloys, which exhibit good magnetic properties, are available for use as a rare earth magnet alloy. For production of R-T-B-based alloys, strip casting methods are widely used.
The production of R-T-B-based alloys by a strip casting method may be carried out by the following procedure, for example.
(a) A molten R-T-B-based alloy is prepared by loading raw materials into a crucible and heating and melting them;
(b) The molten alloy is supplied, via a tundish, to the outer peripheral surface of a chill roll having a structure in which coolant circulates, and quenched. Thus, the molten alloy is solidified to be cast into a ribbon;
(c) The cast ribbon is crushed into alloy flakes; and
(d) The produced alloy flakes are cooled.
The above operations (a) to (d) are usually carried out under reduced pressure or in an inert gas atmosphere to prevent oxidation of the R-T-B-based alloy.
R-T-B-based alloys produced by such a procedure have an alloy crystal structure in which a principal phase and an R-rich phase coexist. The principal phase is a crystalline phase of an R2T14B phase, and the R-rich phase is enriched with the rare earth metal. The principal phase is a ferromagnetic phase that contributes to magnetization, and the R-rich phase is a non-magnetic phase that does not contribute to magnetization.
R-T-B-based alloys can be used as a material for sintered magnets and bonded magnets. In particular R-T-B-based sintered magnets, which have a high energy product ((BH) max) and a high coercive force (Hcj), are used in a variety of applications.
R-T-B-based sintered magnets may be produced by the following process, for example.
(1) Alloy flakes of an R-T-B-based alloy are hydrogen decrepitated (coarsely pulverized), and then are finely ground in a jet mill or the like to form a fine powder;
(2) The produced fine powder is pressed in a magnetic field into a green body; and
(3) The pressed green body is sintered in a vacuum and then the sintered body is heat treated (tempered), whereby R-T-B-based sintered magnets can be manufactured.
In recent years, there has been an increasing need for R-T-B-based sintered magnets, which are manufactured in this manner, to have a higher coercive force. To address this need, efforts are being made to improve the magnetic properties by adding Ga to an R-T-B-based alloy in an amount of about 0.05% to 0.2% by mass. By using a Ga-containing R-T-B-based alloy as a material to manufacture sintered magnets, it is possible to improve the coercive force of the sintered magnets without decreasing their energy product.
With regard to the addition of Ga to an R-T-B-based alloy for sintered magnets, there are various conventional proposals as disclosed in Patent Literatures 1 to 8, for example. Patent Literature 1 relates to an R—Fe—Co—B—Ga-M-based sintered magnet and specifies the amount of Ga to be added. Patent Literature 1 discloses that the addition of Ga improves the coercive force, stating that the reason for this is that the Curie temperature is increased in the BCC phase, which is a soft magnetic phase present at the grain boundaries of an Fe—Co—B—Ga-M-based sintered magnet, so that a significant pinning effect is produced.
Patent Literature 2 relates to an R—Fe—Co—Al—Nb—Ga—B-based sintered magnet, Patent Literature 3 relates to an R—Fe—Nb—Ga—Al—B-based sintered magnet, and Patent Literature 4 relates to an R—Fe—V—Ga—Al—B-based sintered magnet. Patent Literatures 2 to 4 disclose that the balance of the magnetic properties is supplemented by including Dy, a heavy rare earth element, as a technique for improving the coercive force without compromising the energy product.
However, in the actual manufacturing of Ga-containing R-T-B-based sintered magnets, variations in magnetic properties are observed in manufactured sintered magnets, and this poses a problem. Possible causes of the variations in magnetic properties of Ga-containing R-T-B-based sintered magnets are the variations that occur during the process of manufacturing sintered magnets such as for example: variations in elemental diffusion in sintering and heat treatment; or variations in the ground fine powder among the lots. However, there have been a number of uncertainties as to the influence of Ga over the alloy microstructure of a Ga-containing R-T-B-based sintered magnet, and there remains a need to reduce the variations in magnetic properties.
Patent Literature 5 relates to an R-T-B-based sintered magnet and discloses that: the residual flux density and coercive force of a sintered magnet are increased by including a region having a concentrated heavy rare earth element RH at the interface between the principal phase and the R-rich phase of the sintered magnet. Patent Literature 5 discloses Ga as an element that may be added to the R-T-B-based alloy.
Patent Literature 6 relates to an R-T-B-based sintered magnet, and discloses a sintered magnet that includes, on its surface, an amorphous phase-containing layer containing a rare earth metal and oxygen disposed to cover an R-rich phase and thereby exhibits sufficient corrosion resistance even at elevated temperatures. Patent Literature 6 discloses Ga as an element that may be added to the R-T-B-based alloy.
Patent Literature 7 relates to an R-T-B-based magnet material alloy and discloses that: by using an R-T-B-based alloy including a Dy-rich region near its R-rich phase, sintered magnets having a higher coercive force can be manufactured. Patent Literature 7 discloses a Ga-containing R-T-B-based alloy.
However, Patent Literatures 5 to 7 disclose nothing about the advantageous effect of the addition of Ga and its influence over the alloy crystal structure in an R-T-B-based alloy that serves as a magnet material.
Patent Literature 8 discloses casting of an R-T-Q-based magnet material alloy (Q is at least one element selected from the group consisting of B, C, N, Al, Si and P) in which: a molten alloy is quenched to a temperature between 700° C. and 900° C. to be solidified and then is thermally maintained at 700° C. to 900° C. for 15 to 600 seconds, and thereafter is cooled to 400° C. or less. It is stated that this allows heavy rare earth elements such as Dy to be diffused to the principal phase from the grain boundaries, thereby providing the advantage of an increased coercive force achieved by the heavy rare earth elements such as Dy without the need to heat treat the solidified alloy that has cooled to around room temperature. Patent Literature 8 discloses Ga as an element that may be added to the R-T-Q-based alloy. However, Patent Literature 8 discloses nothing about the influence of the microstructure with Ga on the coercive force in the alloy crystal structure.